The integration of renewable energy into the main power supply network challenges the grid, since it was designed for central power production. Electricity generated by renewable energy sources has an unlimited precedence for power supply to the electricity network. Energy production from renewable energy sources is difficult to forecast and depends on weather conditions such as wind speed or solar radiation. To handle this fluctuating production, renewable energy sources need to be curtailed, fossil fueled power plants providing base load need to become more flexible or the excess energy has to be sold cheaply abroad. In addition, the location of production of renewable energy such as onshore and offshore wind does not coincide with the region of high power consumption. Therefore, energy storages play an important role in the improvement of the stability of power supply networks.
Sensible thermal storages are state of the art for storing fluctuating energy from renewable sources. Electrical excess energy from the main supply grid is transformed into thermal energy and is stored in some storage material. In times with no or low occurrence of wind, the stored thermal energy is used for generating steam to produce electrical energy over a steam turbo generator and the produced electricity is fed in the main supply grid.
A possible solution to this problem is provided by a thermal energy storage plant, which is a combination of a charge cycle and a discharge cycle, that are both connected to a heat storage unit. The charge cycle comprises, in a closed loop, a fluid transporting machine, e.g. a fan, a heating device, which may be a resistant or inductive heater, or a heat pump fed by the electrical power generated by a renewable energy source or from the electricity grid, and the heat storage unit. The discharge cycle comprises, in a closed loop, the same heat storage unit of the charge cycle, a blower and a water steam cycle. The water steam cycle includes a thermal machine such as a steam turbine and a heat recovery steam generator (HRSG), a boiler, a heat exchanger or an evaporator, for transferring the thermal energy to water to generate steam which is fed to the thermal machine to produce electrical power from an electrical generator connected to the thermal machine.
The heat storage unit is typically filled with solid or bulk materials, for example stones, bricks, ceramics and other solid materials, which have the ability to be heated up and to keep their temperature over a long period of time in order to store the thermal energy which has been transferred to them. Alternatively a phase change material can be used in the heat storage unit.
These materials are heated using a working fluid, e.g. air, circulating in the charge cycle, which has a temperature higher than the storage material. In the discharge cycle the stored energy is recovered through a flow of the same or different fluid, which has a lower temperature than the storage material. Therefore, the heat storage unit has a respective hot and a cold end.
In the charge cycle, the heat storage unit is connected by a pipe or ducting system to the heating device and to the fluid transporting machine. The fluid transporting machine moves the working fluid through the heating device to the hot end of the thermal storage. A temperature front travels through the heat storage unit from the hot end to the cold end. The temperature front is a zone of strong temperature gradient in the heat storage unit, which separates the hot and the cold zone in the heat storage unit. The charging of the heat storage unit stops, when the temperature at the cold end begins to rise above a chosen temperature threshold.
In the discharge cycle the mass flow of the working fluid will be guided through the heat storage unit in the opposite direction compared to the charge cycle. In the discharge cycle the working fluid enters the heat storage unit at the cold end, reaches the assigned temperature inside the heat storage unit and leaves at the hot end before the working fluid enters the steam generator.
The temperature front travels in reverse direction compared to the charging cycle through the heat storage unit. When the temperature at the hot end begins to decrease the discharging process is stopped.
In the thermal energy storage plants described above, natural convection plays an important role when the heat storage unit is installed horizontally. This is caused by different densities of the working fluid having different temperatures at the hot and cold ends of the heat storage unit. This effect causes a non-homogenous temperature distribution over the length of the storage.
The air at the cold end of the storage has a higher density than the air at the hot end. When the storage plant is in idle mode, between charging and discharging operations, this causes, by natural convection, an air circulation inside the heat storage unit which makes the temperature front tilt: hot air from the upper part of the hot end of the heat storage unit moves towards the upper part of the cold end while cold air from the lower part of the cold end of the heat storage unit moves towards the lower part of the hot end. The longer the storage remains in idle mode, the higher is the air circulation caused by natural convection. This leads to a reduced mixed temperature in the storage, which causes an exergy loss and strongly reduces the usable energy in the water steam cycle and consequently the round trip overall efficiency of the storage plant.
A possible solution may be to use vertical heat storage units, where natural convection plays no considerable role. The temperature front in vertical heat storages is in fact perpendicular to the direction of gravity. Therefore, the temperature front moves vertically through the storage and the temperature front remains unaffected by natural convection during charging, idle or discharging. However, the assembly and installation of vertical heat storage units determines a number of inconveniences, e.g. it causes high cost because of its foundation requirements, inlet and outlet connections are more complex and heights available for the installation may be limited.
Therefore, it may be a need for improving a thermal energy storage plant in such a manner that the above mentioned inconveniences can be suppressed or reduced in an optimized way.